1. Field of the Invention
The present invention relates to an intake manifold for an engine, and more particularly to a structure of a branch pipe thereof.
2. Description of the Related Art
Conventionally, in an intake manifold for an engine, each branch pipe 3 connected to an intake port 2 of a cylinder head 1 has an opening 4 on the intake port 2 which has an inner diameter almost equal to that of the intake port 2. The inner diameter of the branch pipe 3 gradually increases toward the other opening 6 connected to a surge tank 5, as illustrated in FIG. 5. The intake air amount of cylinder in the engine or volumetric efficiency is affected by the length and inner diameter of the branch pipe 3.
That is, as the inner diameter of the branch pipe 3 in the intake manifold increases and the length of the branch pipe 3 decreases, the volumetric efficiency at the time of high speed of engine is increased to produce a higher output.
And as the inner diameter of the branch pipe 3 in the intake manifold decreases and the length of the branch pipe 3 increases, the volumetric efficiency at the time of medium speed or low speed of engine is increased to produce a higher output.
Accordingly, to increase the volumetric efficiency at the time of medium speed and low speed of engine, there is conventionally a case in which a mechanism for varying the length of the branch pipe according to the rotating speed of the engine is provided. But when the length of the branch pipe 3 is fixed as illustrated in FIG. 5, it is required to change the inner diameter of the branch pipe 3 according to the distance L from the opening 4 on the intake port in the branch pipe 3. Ideally, to obtain the excellent volumetric efficiency according to the engine characteristics, it is necessary to change the inner diameter of the branch pipe 3 according to the distance L from the opening 4 as shown in FIG. 6 in which the medium speed and high speed specifications are illustrated.
However, if the inner diameter of the branch pipe 3 in the engine is set in either the medium speed or high speed specifications for the engine, the volumetric efficiency at the time of high speed is greatly decreased for the branch pipe 3 in the medium speed specifications, or the volumetric efficiency at the time of medium speed is remarkably decreased for the branch pipe 3 in the high speed specifications, as represented by a curve x in the medium speed specifications or a curve y in the high speed specifications in FIG. 7.
In JP-A-10-213025, an intake manifold was disclosed in which a branch pipe has a constant inner diameter, and has a greater diameter near a connection portion with a surge tank. With this constitution, the intake air flow efficiency from the surge tank into the branch pipe is increased. However, with a technique as described in this publication, since the inner diameter of the branch pipe is constant from the intake port connection portion to the vicinity of the surge tank, the inner diameter of the branch pipe is almost equal to the diameter of the intake port. Accordingly, it is possible that the branch pipe of this shape has a greater branch pipe sectional area than the branch pipe having the same length, as illustrated in FIG. 5. Thereby it has the increased volumetric efficiency at the time of medium speed and the increased intake air flow efficiency. But there is a problem that the volumetric efficiency at the time of high speed is not improved.